1. Field of the Invention
The present invention relates to a crystal growing apparatus which is used for manufacturing a single crystal or the like used as a semiconductor material.
2. Description of the Prior Art
There are various methods of manufacturing a silicon single crystal, and a typical one is a Czochralski method (CZ method). In the manufacture of silicon single crystal by the CZ method, as known, a seed crystal is immersed in a silicon melt formed in a quartz crucible, and from this condition, the seed crystal is gradually pulled upward while rotating the crucible and seed crystal to thereby grow the silicon single crystal beneath the seed crystal.
In the pulling of silicon single crystal by such a CZ method, it is known that defect distribution or the like in a crystal section depends on a crystal growing rate, that is, pulling rate. Specifically, the more the pulling rate is increased, the closer to a periphery a ring OSF generating area moves, and is finally eliminated from a crystal effective portion. On the other hand, the less the pulling rate is reduced, the closer to the center of the crystal the ring OSF generating area moves, and finally disappears at the center.
An outside and inside of the OSF generating area are both defect generating areas, but kinds of defects are different between the outside and inside. It is also known that an increased pulling rate surely improves productivity with smaller defects. For this reason, increasing the pulling rate is pursued as one direction of crystal growing.
Installing a heat shield is known as a technique for achieving the increased pulling rate. The heat shield is an inverted frusto-conical tubular heat insulating member provided so as to surround the single crystal, and blocks the melt in the crucible or radiant heat from a heater arranged outside the crucible, thereby facilitating cooling of the single crystal pulled from the melt to achieve the increased pulling rate.
A technique for installing a tubular cooler which is forcedly cooled by water inside a heat shield has been also noted lately (Japanese Patent Laid-Open No. 63-256593, Japanese Patent Laid-Open No. 8-239291, Japanese Patent Laid-Open No. 11-92272, Japanese Patent Laid-Open No. 11-292684). The tubular cooler which is forcedly cooled by water is installed inside the heat shield so as to surround a single crystal, thereby further facilitating cooling of especially hot portions of the single crystal to achieve more increased pulling rate.
In the growing of single crystal by the CZ method, an inside of a furnace is brought into a reduced pressure atmosphere. Also, there is a possibility of damage to various hot zone constituting members including the crucible due to accidental drop or the like of the single crystal.
Among members for achieving the increased pulling rate, damage of the heat shield does not cause very serious problems. However, when the cooler is damaged, there is a possibility of leakage and rapid expansion of cooling water flowing through the cooler in the furnace thereby to induce steam explosion. Even when the cooler is not damaged, there is a possibility of spontaneous leakage of the cooling water due to secular changes or the like.
Crystal growing apparatuses using coolers which have been proposed in the past have serious problems in safety operation since no means for detecting leakage from the cooler are mounted.
The present invention has its object to provide a crystal growing apparatus with a high level of safety which achieves an increased pulling rate by a cooler and prevents serious accidents due to leakage from the cooler.
In order to attain the object, the present invention is directed to a crystal growing apparatus which grows a single crystal from a melt by a CZ method, including: a tubular cooler which is provided so as to surround the single crystal grown from the melt, and is forcedly cooled by cooling water flowing through the cooler to cool the single crystal; and flowmeters which are provided on a cooling water inflow side and a cooling water outflow side of the cooler, and measure flow rates of the cooling water on the respective sides.
In the crystal growing apparatus of the present invention, when no leakage occurs in the cooler, an inflow rate of the cooling water into the cooler coincides with an outflow rate of the cooling water from the cooler. When the leakage occurs in the cooler, the outflow rate from the cooler is reduced compared with the inflow rate into the cooler, and the flow rate difference is detected from outputs of the flowmeters provided on the inflow side and outflow side.
The flow rate difference corresponds to a leakage amount from the cooler. Studies by the inventors of the present invention reveal that slight leakage from the cooler does not have large influence on furnace pressure, and that there is a high possibility that the furnace pressure is rapidly increased when the leakage amount exceeds a predetermined value, specifically 20 cc/second, leading to steam explosion. Therefore, preferable flowmeters are capable of measuring the flow rates under 20 cc/second.
By providing abnormality detecting means which uses a threshold value of the flow rate difference of 20 cc/second and determines abnormal leakage when the flow rate exceeds 20 cc/second, the abnormal leakage only is surely selectively detected.
As a remedy when the abnormal leakage is detected, automatic valves respectively provided on the cooling water inflow side and cooling water outflow side of the cooler are preferably operated by an abnormality detecting signal output from the abnormality detecting means. This permits water supply to the cooler to be automatically stopped when the abnormal leakage occurs, and also permits prompt drain of water from the cooler.
Especially, the drain of water from the cooler can be promptly carried out when the automatic valve on the inflow side is an open/close valve for opening and closing a water supply pipe, and the automatic valve on the outflow side is a switch valve for switching a pipe line from a usual drain pipe to an emergency drain pipe.
The flowmeter is preferably arranged near the cooler, specifically within 3 m along a pipe length of the cooling water in order to reduce influence of pipe resistance or pulses of water flow.
The cooler is made from a metal which is forcedly cooled by passing water. Preferable metal is a copper based metal containing copper with high thermal conductivity as a base. A specific dimension and shape of the cooler is appropriately designed depending on pulling conditions.
The cooler is also preferably combined with a heat shield and arranged therein. Combination with the heat shield facilitates cooling of the crystal, and also more effectively restrains an increase in temperature of the cooler itself to promote an increased pulling rate.